UBC Theses and Dissertations
The reduction of cupric salts in aqueous solution by molecular hydrogen Macgregor, Edwin Robert
The thesis describes a kinetic study of the reactions between cupric salts and hydrogen in aqueous solution. The first part is concerned with the homogeneous activation of hydrogen in solution by cupric perchlorate, as evidenced by its catalytic effect on the reaction between dichromate and hydrogen. These studies, which were conducted on solutions of higher acidity than previously investigated, provide further support for the mechanism proposed for this reaction, i.e., Cu⁺⁺ + H₂[formula omitted] CuH⁺+ H⁺ CuH⁺ + Cu"⁺⁺" □(→┬(k₂) ) 2Cu⁺ + H⁺ 2Cu⁺ + substrate □(→┬fast ) 2Cu⁺⁺ + products and the corresponding rate equation: (d[H^2])/dt = (k^₁ [Cu^(++) ]^2 [H^2])/((k^(-1)/(k^2 [H^+ ] )+[Cu^(++)]) The second part deals with the thermodynamics and kinetics of the precipitation of metallic copper from aqueous solution by H₂. The reduction reactions were studied in both cupric perchlorate and cupric sulphate solutions and the effects of a number of variables on the reaction rate in both systems are compared. The rate, which was initially fast, decreased gradually until an apparent steady-state condition was approached. This behavior was common to both systems, although the rates were much higher and the. residual copper concentration lower in the sulphate system. This apparent steady-state is not predicted by thermodynamics and is considered to result from kinetic factors. The dependence of the reaction rate on a number of variables can be predicted from the mechanism Cu⁺⁺ + H₂[formula omitted]CuH^++H⁺ CuH⁺ + Cu⁺⁺□(→┬(K₂) ) 2Cu⁺ + H⁺+ + 2Cu² □(→┬fast ) Cu° + Cu⁺⁺ which corresponds to an overall reaction of Cu⁺⁺ + H₂ → Cu° + 2H⁺ Reasonably good quantitative agreement between the predicted and measured kinetic dependence on Cu⁺⁺, H⁺ and H₂ concentration suggests that the rate of reduction of cupric salts is determined by the homogeneous activation of H₂ by Cu⁺⁺+ or a cupric complex. Although the initial decrease in rate can be predicted by this mechanism, the apparent steady-state condition approached cannot be fully explained. The equilibrium constant for the reaction 2Cu⁺ ⇌ Cu⁺⁺ + Cu° was also determined experimentally at several temperatures and compared with calculated values.
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